Authenticate with Firebase using a custom authentication system and Unity

You can integrate Firebase Authentication with a custom authentication system by
modifying your authentication server to produce custom signed tokens when a user
successfully signs in. Your app receives this token and uses it to authenticate
with Firebase.

Authenticate with Firebase

Call Firebase.Auth.FirebaseAuth.SignInWithCustomTokenAsync with the token from
your authentication server.

When users sign in to your app, send their sign-in credentials (for
example, their username and password) to your authentication server. Your
server checks the credentials and returns a
custom token
if they are valid.

After you receive the custom token from your authentication server, pass
it to Firebase.Auth.FirebaseAuth.SignInWithCustomTokenAsync to sign in the
user:

Next steps

After a user signs in for the first time, a new user account is created and
linked to the credentials—that is, the user name and password, phone
number, or auth provider information—the user signed in with. This new
account is stored as part of your Firebase project, and can be used to identify
a user across every app in your project, regardless of how the user signs in.